under one's breath

Idioms  

1. Softly, in an undertone or whisper, as in “I can't stand one more minute of that music,” she muttered under her breath. This idiom, first recorded in 1832, is probably a hyperbole, alluding to a sound that is softer than breathing.

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Example Sentences

Here, of course, one can only speak of Calvin under one's breath, but some day, when we are more private," and Mrs. Church looked round the room, "I will give you my view of him.

From The Pension Beaurepas by James, Henry